The relativistic effects for particles in external laser fields are controlled by a Lorentz-invariant parameter \(a_0\) , so-called unitless normalized laser amplitude [1]}, given by
\(a_0 = \frac{e\sqrt{|A^\mu A_\mu |}}{mc} = \frac{e\mathcal {E}}{mc^2} = 1.2 \frac{\sqrt{I[10^{20} \text{ W/cm}^2]}}{\hbar \omega [1 \text{ eV}]}\,,\) 
